1. Field of the Invention
This invention relates to a catalyst for nitrogen oxide removal, which removes nitrogen oxides (NOX) in an exhaust gas discharged from a boiler or the like, and an exhaust gas treatment method. More specifically, the invention relates to a catalyst for nitrogen oxide removal, which is useful in treating an exhaust gas having a high proportion of nitrogen dioxide to nitrogen oxides, and an exhaust gas treatment method.
2. Description of the Related Art
An exhaust gas discharged from a boiler or the like contains nitrogen oxides (NOX), and most of (90% or more) of the nitrogen oxides are present as nitrogen monoxide (NO). As a denitration method for such an exhaust gas, practical use is made of an ammonia catalytic reduction process by which ammonia (NH3), as a reducing agent, is poured into (added to) the exhaust gas, and this exhaust gas is brought into contact with a catalyst having vanadium oxide and tungsten oxide supported on a titania carrier, whereby NOx is decomposed into harmless nitrogen and water.
With an exhaust gas from a gas turbine, a diesel engine, a gas engine, or a chemical plant such as a nitric acid plant, which greatly changes in load, however, there is a case where the proportion of nitrogen dioxide (NO2) is higher than the proportion of NO. In treating such an exhaust gas, the treatment of the exhaust gas with the above-mentioned conventional catalyst results in a decreased denitration rate.
Various catalysts containing chromium have been proposed as catalysts showing sufficient denitration performance even for an exhaust gas having a higher proportion of NO2 than the proportion of NO as stated above (see, for example, Japanese Unexamined Patent Publication No. 1998-118492).
However, the above-mentioned catalyst has sufficient denitration performance for an exhaust gas having a higher proportion of NO2 than the proportion of NO, but there is a concern about chromium contained in this catalyst. Thus, a demand for a chromium-free catalyst is expressed.
The present invention has been proposed in light of the above-described circumstances. It is an object of the invention to provide a catalyst for nitrogen oxide removal which can show sufficient denitration performance for an exhaust gas having a higher proportion of NO2 than the proportion of NO, although the catalyst contains no chromium, and to provide an exhaust gas treatment method.